This invention relates generally to screw pumps and more particularly to sealless screw pumps for multi-phase undersea pumping from offshore oil wells, for surface platform mounting at such wells, and for high pressure pumping of single-phase viscous fluids.
Screw pumps usually consist of two or more oppositely handed parallel screws or augers with intermeshed flights which rotate within a pumping chamber to create a number of axially moving sealed pockets between their flights. These pockets transport product from the suction port to the discharge port of the pump. Sealing discharge pressure from suction pressure is accomplished by the extent of the radial clearance between the screws and the mating bore as well as by the locking of the intermeshed flights. Their mechanical simplicity, reliability, and compactness provide significant value to users. Multiphase fluids such as mixtures of gas and oil are easily accommodated by rotary screw pumps.
Typically, screw pumps are equipped with a set of timing gears for transmitting torque from a single drive motor to both screws. One screw has an extended shaft that is coupled to the drive motor, such that torque from the drive motor is transmitted through the shaft to a set of the timing gears to synchronously drive both screws. The timing gears serve to avoid potentially damaging contact between the screws; however, they require an oil system for proper lubrication to avoid damage to the timing gears themselves. A shaft sealing arrangement is also required to prevent infiltration of the working fluid into the lubricating oil and loss of lubricating oil. The drive motors are usually induction motors which are sealed for undersea applications and explosion proof for surface applications.
In undersea duty, the sealed motor is typically cooled by seawater, which requires that both the motor and the coupling to the extended screw shaft be sealed from the pumped product as well as the surrounding seawater. Alternatively, motor cooling can be provided by the oil system of the timing gears via the rotor/stator interface of the motor. The use of shaft seals, oil systems, timing gears, and mechanical couplings introduce significant mechanical complexities which adversely affect reliability and cost. Moreover, any repair to a sea bottom pump is very expensive in terms of downtime and the cost of specialized recovery and repair equipment.
The foregoing illustrates limitations known to exist in present single-phase and multi-phase screw pumps. Thus, it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.